15 kV AC

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Image:BSTROM1.jpg 15 kV AC at 16.7 Hz is a railway electrification system chosen by Germany, Austria, Switzerland, Sweden, and Norway in 1912. The high voltage enables high power at low losses, while the lower frequency reflects technical limitations at the turn on the 20th century. In particular, the lower frequency reduced flashover problems in the motors, albeit at the cost of a non-standard line frequency requiring frequency conversion, and separate supplies.

In Sweden it is officially 16 kV AC, 16 2/3 Hz.

This voltage has been superseded by 25 kV AC at 50 Hz or 60 Hz since the 1950s.

Oddly, when Denmark decided to electrify in the 1970s they chose not to use 15 kV AC, which would have linked compatible systems in adjacent Germany and Sweden, but rather 25 kV AC.

The lower frequency of 16.67 Hertz requires either the installation of a special power grid for single phase AC or the usage of substations with rotary and/or static inverters. In Germany (excpept Mecklenburg-Vorpommern and Saxony-Anhalt), Austria and Switzerland, there is a special power grid for single phase AC current with 16.7 Hertz. The voltage of this grid is 110 kV in Germany and Austria and 66 kV, 132 kV in Switzerland.

At these powerlines the middle is grounded via a coil, so each conductor of a 110kV powerline for single phase AC has a voltage of 55 kV against ground. The coil over which the grounding is done, is so dimensioned that it compensates ground error currents in case of a defect of the line.

For this grid in several power stations in Germany, Austria and Switzerland, there are special generators in some powerstations, like the Walchensee Powerstation or the nuclear power plant in Neckarwestheim. There are and were also powerstations exclusively producing single phase AC with a frequency of 16.7 Hertz. Also in use are central plants with rotary machines (and also with static inverters) for the transforming of three phase AC with 50 Hertz of the national power grid into 16.7 Hertz for the railway. Such plants are located in some power stations and on big railway nodes.

Substations in areas where the railway is fed by a single phase AC powerline consist of a switchyard and a transformer for transforming the voltage of the grid (66 kV, 110 kV or 132 kV) into those of the overhead line (15kV).

In Mecklenburg-Vorpommern, Saxony-Anhalt, Sweden and Norway, there are inverters at the substations. Most of them use rotary machines, consisting of a three phase AC-machine and a single phase AC-machine for transforming three phase 50 Hertz AC of power grid into 16.7 Hertz single phase AC for the rail.